While gas, such as ozone, oxygen, carbon dioxide, chlorine, etc., is to be injected into a liquid through a gas injection device, such as a negative pressure type (such as a Venturi tube) or a positive pressure type (such as a diffuser), there must be a check valve/check valves in the injection device to prevent back flow. The conventional types of check valve include the spring-gas balanced pressure type, the back-flow water pressure surface closure mechanical switch type, etc. The former is highly sensitive to pressure change; thus it may have the disadvantage of blocking fluid flow. Further, it is frequently subject to malfunction due to the fatigue of the spring or the pressure fluctuation in the gas injection device, whereas the latter is frequently subject to malfunction resulting from a defect in mechanical manufacture and structure. In addition, the back-flow leaking liquid from the check valve structure may gradually accumulate in the gas injection device and cannot be discharged smoothly, therefore hampering normal operation of the gas injection device.
Accordingly, the liquid back flow through the gas outlet conduit and the discharge of the back-flow liquid accumulated in the gas injection device are the serious problems that need to be overcome.